The present invention relates generally to millimeter wave and submillimeter wave signal receiver systems, and more specifically the invention pertains to a means for receiving and detecting characteristics of such radio frequency signals by heterodyning signals. Heterodyning signals entails mixing two frequencies together to produce another frequency which can equal the sum or the difference of the first two frequencies. The difference frequency is called intermediate frequency (IF).
Millimeter wave radio frequency signals are those signals which are greater in frequency than 30 GHz.
The task of providing a selfmixing heterodyne system to enable an IF detector to coherently detect characteristics of millimeter and submillimeter wavelength signals is alleviated, to some extent, by the systems disclosed in the following U.S. patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 4,830,479 issued to Lammers et al; PA0 U.S. Pat. No. 4,910,523 issued to Huguenin et al; PA0 U.S. Pat. No. 2,934,756 issued to Kalmus; PA0 U.S. Pat. No. 4,709,237 ussed to Poullain et al; PA0 U.S. Pat. No. 4,075,632 issued to Baldwin et al.
The Lammers et al reference discloses the rotating Doppler frequency shifter used in the present invention. The Huguenin reference, if used as a point-to-point or radar imaging receiver, can be improved by the present invention. It eliminates the need for separate transmit and local oscillator (LO) sources. The remainder of the references cited involve homodyne reception techniques. Homodyne reception is closely related to the present invention, in that it also uses a single source for transmit and LO signals. Since neither signal is being shifted in frequency before selfmixing, the IF is zero. Skolnik in his Radar Handbook, published by McGraw-Hill Book Company, addresses homodyne systems in Chapter 16.9, Miscellaneous CW Radars. The coherence of a homodyne IF signal is as good as that of a selfmixing heterodyne system. However, owing to the lack of a frequency offset between the transmit and LO signals, spectrum foldover may result to a modulation imposed on the transmit signal The Kalmus reference requires quadrature mixers to avoid spectrum foldover Also, an IF of zero leads to a degraded sensitivity in a homodyne receiver Poullain et al describe a method to reduce low frequency noise in a homodyne radar receiver. Spectrum foldover and sensitivity degradation can both be avoided in a selfmixing heterodyne receiver. The selfmixing heterodyne method is suitable to improve other homodyne systems such as the one described in the Baldwin et al reference.
Source power for laboratory-type experiments is often limited at millimeter and submillimeter wavelengths. This requires the use of sensitive receiving equipment, such as a narrowband heterodyne detector. High receiver sensitivity and narrowband coherent resolution can be achieved through the process of heterodyning or IF generation, where the two frequencies, whose IF is being generated, need not themselves be very stable in frequency and phase, if their fluctuations cancel at the IF. This can be achieved by deriving both from one source. We have applied this concept twice in a millimeter or submillimeter receiver of high sensitivity and resolution. At the same time the receiver is simple, since the frequency stability of the two sources involved does not affect sensitivity and resolution.
This invention pertains to a well-defined subrange out of a wide range of millimeter and submillimeter system tasks; namely the sensitive and coherent reception of signals of moderate bandwidth, where these signals are generated locally, then amplitude, frequency, or phase modulated by the device or process under test and finally received. A large number or laboratory-type experiments fall into this category. The present invention enables IF detectors to detect signal characteristics of millimeter wavelength signals in all of the applications discussed above.